The present invention relates to the application of novel extraction procedures to hydrometallurgical methods for recovery of metals from aqueous solutions using membranes with metal ion diffusivity in the range of about 10.sup.-.sup.5 to about 10.sup.-.sup.8 cm.sup.2 /sec.
Liquid-liquid extractions have been a useful means for the separation of various materials. In such systems, in general a liquid solvent C is used to extract a solute (or solutes) from a second liquid B in which the solute is dissolved. Liquids B and C are immiscible or partially miscible.
For a conventional operation, liquids B and C are mixed directly, then separated into two phases. During the direct mixing, however, certain inherent difficulties are present. To illustrate, a foam frequently develops which has the disadvantages, for example, of reducing the mass-transfer rate and prevents a complete phase separation. To increase mass-transfer area in conventional solvent extraction, small drops have to be formed during the mixing. In such cases, the power consumption in processes involving a mixture of the liquid phase is high due to such operations such as mixer-settler, mechanical stirred column, etc. When the drops which occur during the mixing are formed, a back mixing may occur, leading to a reduction of mass-transfer rate. Also, the smaller the size of the drops, the more readily a foam is generated.
In a given system, if the metal is to be extracted from a mixture of solutes, the liquid solvent C, for a conventional operation, is chosen so as to extract the desired metal selectively from other solutes in the liquid B. However, sometimes the selectivity becomes difficult to establish. In addition to the limitations of the conventional solvent extraction mentioned above, solvent loss is high owing to entrainments and hold-up tanks are generally required for phase separations. The present invention, using a membrane system, avoids these and various other drawbacks inherent in conventional solvent extraction techniques.